1. Field of the Invention
The present invention relates generally to air quality sensors and, more specifically, to a method of establishing a filtered baseline signal for an air quality sensor.
2. Description of the Related Art
Air quality sensors (AQS) are sensors that sense noxious gases. Such sensors can be used by a climate control system of a motor vehicle to shut off intake air into an occupant compartment of the motor vehicle, thus preventing pollution and undesired smells from reaching occupants in the occupant compartment. Typically, the air quality sensors use a heated thin film, which reacts with certain gases and changes resistance. These air quality sensors generally fall into one of two categories: reducing or oxidizing, according to the kind of gases they can detect. Reducing air quality sensors detect such gases as unburned hydrocarbons in diesel exhaust, carbon monoxide, methane from cattle in feedlots, etc. Oxidizing air quality sensors detect such gases as nitrogen oxides, ozone and chlorine.
The signal from the air quality sensors can be related to absolute concentrations of particular gases, but because the sensors detect such a wide variety of gases and are exposed in use to an unpredictable mix, comparing the signal to any absolute level is not very useful. Rather, it is desirable to take advantage of the transient characteristics of typical signals obtained from these air quality sensors on a moving motor vehicle. Most of the time, the air quality sensor will produce a background signal level characteristic of the regional air mass the motor vehicle is moving through. When the motor vehicle passes through a plume from a point source of detectable gases, a spike in the signal will occur and then fade away as the motor vehicle moves out of the plume. The background signal level will slowly change as the motor vehicle moves from one region to another such as from rural to urban areas.
It is desirable to provide a baseline filter for air quality sensor signal processing. It is also desirable to provide a baseline filter for establishing a moving baseline for the background signal level and take action when the signal level exceeds a certain threshold above this baseline for an air quality sensor. It is further desirable to provide a method that tracks the baseline of a signal from an air quality sensor without unduly following spikes. Therefore, there is a need in the art to provide a method of establishing a baseline filter for an air quality sensor.